"When someone visits a primary care provider or a dermatologist regarding dandruff, we need to do an examination of the scalp."

Mayo Clinic dermatologist Dr. Dawn Davis says there are two common causes for dandruff flakes on the scalp.

"We can differentiate whether the patient has overactive oil glands, simply dryness and dehydration causing scaling and flaking, or a combination thereof."

You can treat both types of dandruff with special shampoos that have active ingredients designed to reduce irritation and rehydrate the scalp.

"Be careful and aware how you use your shampoo." Dr. Davis says most dandruff shampoos need to be left on the scalp for more than five minutes.

"Shampoo has a precipitate that once applied takes several minutes to separate itself out to allow it to remain on the skin to become active."

And Dr. Davis says dandruff-fighting routines can become less effective over time, so for best results she recommends rotating among up to three shampoos with different active ingredients."

For the Mayo Clinic News Network, I'm Jeff Olson.